1. Field of the Invention
The present invention relates to a two-dimensional light-emitting device using LD (laser diode) arrays, and in particular to a two-dimensional LD array light-emitting device constituted by stacking a plurality of planar light-emitting units each having a LD bar and a cooling assembly for cooling the LD bar.
2. Description of Related Art
In recent years, a surface light-emitting device using semiconductor laser diodes is drawing attention for use as a pumping light source in a solid-state laser generator such as YAG laser because of a high pumping efficiency. The light source device using LD is advantages in its reduced size and long life in comparison with a conventional discharge lamp such as a xenon lamp.
In constituting a surface light-emitting device using the laser diodes, one-dimensional LD arrays having light-emitting regions aligned linearly are used. The one-dimensional LD array is generally called as “LD bar” since it has a shape of a bar. The LD bars are arranged to form the two-dimensional LD array light-emitting device (surface light-emitting device).
The laser diodes in the LD bar generate considerable heat when driven to raise temperature of the LD bar. Particularly in a case of arranging adjacent LD bars close to each other so as to reduce a dead space (non-emitting region) between light-emitting regions of the LD bars in the surface light-emitting device, there arises a significant problem of how to effectively remove the heat generated by the LD bars. For example, when the two-dimensional LD array is used as a pumping light source of the high-power solid-state laser, an average output power of the surface light-emitting device is 100-200 W/cm2 as a surface light-emitting device with generating heat of 200-400 W/cm2.
In order to absorb the large amount of heat from the LD bars to suppress rising of temperature thereof, it is adopted a structure in which the LD bar is mounted on a planar cooling assembly to be thermally connected therewith. The planar cooling assemblies with the LD bars mounted thereon are stacked to form the surface light-emitting device. A partial cross section of such surface light-emitting device is shown in FIG. 5.
As shown in FIG. 5, the surface light-emitting device comprises a large number, e.g., several hundreds of light-emitting units, of which (n−1)th, n-th, (n+1)th three adjacent light-emitting units Rn−1, Rn and Rn+1 (n: integer not less than two) are shown in FIG. 5.
The n-th light-emitting unit Rn will be described as a representative of the plurality of light-emitting units. A cooling assembly 10 of the light-emitting unit Rn has a laminated structure constituted by three metal plates 11, 12 and 13. An exploded view of the cooling assembly 10 is shown as FIG. 4. Openings 16 and 17 are formed in the metal plates 11, 12 and 13 to form passages for supplying/discharging coolant to/from the cooling assembly 10.
In this example, the opening 16 is used for a passage for supplying the coolant into flow paths 15 in the cooling assembly and the opening 17 is used for discharging the coolant from the flow paths 15. The coolant flows from the opening 16 to the opening 17 through flow paths 15 formed by grooves and openings of the metal plates 11-13.
The path 15a is positioned immediately under the LD bar 56 arranged on the cooling assembly 10. O-rings and rubber sheets (not shown) are provided between the adjacent cooling assemblies 10 for sealing peripheries of the openings 16 and 17 to prevent leakage of the coolant.
The LD bar 56 is mounted on an electrically-conductive die spacer 55 fixed on the metal plate 13 in the vicinity of a peripheral side thereof such that one electrode (e.g., a positive electrode) of the LD bar 56 is electrically connected with the die spacer 55. The other electrode (e.g., a negative electrode) of the LD bar 56 is connected with one end of a bonding wire 53 such as a gold wire. A bonding portion of the wire 53 with the LD bar 56 is indicated by a reference numeral 54.
An insulating sheet 51 is arranged on the metal plate 13 with a predetermined space formed between the insulating sheet 51 and the die spacer 55, and an electrically-conductive connection board 52 is arranged on the insulating sheet 51. The other end of the bonding wire 53 is connected with the connection board 52 at a position not so remote from the LD bar 56. A protrusion 52a of the connection board 52 is electrically connected with a metal plate 11 of a cooling assembly 10 of the adjacent cooling assembly Rn+1. In the similar manner, the metal plate 11 of the cooling assembly Rn is electrically connected with a protrusion 52a of the connection board 52 of the other adjacent cooling assembly Rn−1.
Thus, the metal plate 11 of the cooling assembly 10 serves as one electrode (e.g., positive electrode) of the light-emitting unit Rn and the connection board 52 serves as the other electrode (e.g., negative electrode ultimately connected to the ground) of the light-emitting unit Rn. A number of light-emitting units are connected in series such that a driving current flows the respective LD bars in series. The cooling assembly 10 of the first light-emitting unit R1 and the connection board 52 of the final light-emitting unit RN are connected to a positive terminal and a negative terminal, respectively, and vice versa of an electric power source.
Openings respectively corresponding to the openings 16 and 17 of the cooling assemblies 10 are formed on the insulating sheet 51 and the connection board 52 so that continuous passages of the coolant are formed through the stacked cooling assemblies 10. One end or both ends of the passage for providing the coolant and one end or both ends of the passage for discharging the coolant are connected to an inlet and an outlet, respectively, of a circulation pump.
With the stacked structure of the surface light-emitting device, a manufacturing cost of the surface light-emitting device is reduced. The cost of the parts such as bonding parts and assemble cost, and cost for assembling the cooling assemblies to be stacked to form a surface light-emitting device are relatively increased by a ratio thereof in the whole manufacturing cost of the surface light-emitting device. Thus, it has been strongly desired to improve the structure of the surface light-emitting device to enable a cost reduction thereof.
Reviewing the conventional structure in the above view, there is a problem in that the gold wire has to be arranged from the other electrode of the LD bar to the connection board 52 which is insulated from the cooling assembly 10. Thus, the insulating sheet 51, the connection board 52 and the bonding wire 53 are necessary as essential parts for conducting the other electrode of the LD bar with the metal plate 11 of the adjacent light-emitting unit. Since a large number, e.g., several hundreds, of light-emitting units are stacked for forming the surface light-emitting device, cost of the necessary parts and assembling operation thereof increases in dependence of the number of parts.
Further, the above structure of the light-emitting device requires special structure and special parts such as the gold wire to raise the manufacturing cost. If mass-produced parts are used in attempt to reduce the cost of the parts, there arises a problem of restriction of disabling thickness of the surface light-emitting device.